1. Field of the Invention
The present invention is directed to the transmission and reception of information in a wireless communication system and more particularly to a method of interleaving information conveyed over a wireless communication system.
2. Description of the Related Art
Wireless telecommunication systems use various schemes to allow multiple users to use (i.e., share simultaneously) a particular bandwidth. The bandwidth is part of or all of the band of frequencies allocated to the telecommunication system for conveying (i.e., transmitting and receiving) information. The wireless telecommunication systems typically use techniques known as multiple access schemes to allow a plurality of users to share a given bandwidth. One multiple access scheme that is widely used in many wireless telecommunication systems is called Time Division Multiple Access (TDMA).
A TDMA wireless telecommunication system allows multiple users to use the same bandwidth by creating time frames (hereinafter xe2x80x9cframesxe2x80x9d) comprising time slots (hereinafter xe2x80x9cslotsxe2x80x9d) within which each user is allowed to convey their information. Each slot or a group of slots are defined and are assigned to a particular user. Thus, each user is allowed to transmit and/or receive information in that user""s assigned slot. In this manner, many users are able to use the same bandwidth thus increasing the user and/or information capacity of the system.
FIG. 1 depicts a typical scenario in a TDMA system where user 1 is conversing with user 2. For the sake of clarity, only two users are shown. In an actual system, there is usually more than one base station and many more users throughout the system some of whom obtain access to base station 106 via a wireline telephony network such as the Public Switched Telephone Network (PSTN) (not shown). The users have mobile equipment 100 and 112 (e.g., cellular phone) which they use to communicate with each other. Base station 106 represents part of the TDMA system""s equipment which relays the user information in accordance with the protocol used by the system. The protocol is a particular set of rules by which the system equipment and the user equipment initiate communication, convey information and terminate communication. Typically, protocols are established communication standards that are well defined and which are followed by many wireless telecommunication systems. Wireless communication channels 102 and 110 are commonly referred to as the uplink, which is the channel used by the users to transmit information to base station 106. Wireless communication channels 104 and 108 are commonly referred to as the downlink, which is the channel used by the users to receive information from base station 106. The information is conveyed over the uplink and downlink in the form of Radio Frequency (RF) signals. Base station equipment 106 comprises radio equipment (e.g., transmitters and receivers) and other system equipment owned by a system or network provider.
FIG. 2 depicts a particular frame format used by a protocol referred to as the IS-136 standard. The IS-136 standard is a protocol used by many TDMA wireless communication systems in North America and South America. The frame format is the particular arrangement of the slots within frame 200. In particular, frame 200 has a time duration of 40 milliseconds and is divided equally into six slots. Thus, each slot has a time duration of 6.67 milliseconds. For IS-136, the frame format (i.e., six slots per frame) is the same for the uplink and the downlink.
In IS-136, either three (3) users or six (6) users are allowed to convey information within the frame format discussed above. The number of users conveying information per frame depends on the mode at which the TDMA system is operating. In IS-136 compliant systems there are two modes of operation. One mode of operation is referred to as the Full Rate mode in which each user is assigned two slots per frame. The other mode of operation is called the Half Rate mode in which each user is assigned one slot per frame. Thus, there are six users per frame in the Half Rate mode and three users per frame in the Full Rate mode. A particular slot allocation scheme in IS-136 Full Rate mode for the uplink and the downlink has a first user assigned to slots 1 and 4, a second user assigned to slots 2 and 5 and a third user assigned to slots 3 and 6.
TDMA signals, especially uplink signals, are by nature bursty signals; that is the signals for any particular user are transmitted and received as short intermittent bursts each of which carries a relatively great amount of information. The RF signals which carry the user information are exposed to various well known distorting effects such as phase jitter, frequency offset, amplitude and phase fading and multipath distortions. When a transmitted or a received burst of information is adversely affected by channel distortions, a substantial amount of the conveyed information is lost. Therefore, to reduce the likelihood of losing significant amounts of information, a certain amount of processing of the information is done prior to transmission. The intent of the processing is twofold: first it is to introduce time diversity to the information with the use of well known channel coding techniques and second, to spread out in time the burst of information with the well known technique of interleaving. The processing of information to be transmitted to mobile stations and information received from mobile stations is performed by system equipment typically located at base station 106.
Time diversity adds repetitions to the user information to reduce the likelihood of losing information due to channel distortions. The repetitions add redundancy in the information rendering said information more robust (better able to withstand noisy conditions) with respect to a noisy communication channel. The technique of interleaving spreads out a burst of information over a period of time further adding time diversity to the information. Thus an entire slot of information for a user is less likely to be lost since the information is spread across two or more slots. Channel coding introduces time diversity to information within a slot by adding information (e.g., bit paddingxe2x80x94adding repetitive blocks of bits) to the information. Because channel coding actually adds information to the user information to be conveyed, it is costly since it reduces a system""s overall capacity. Further, the time diversity introduced by channel coding is limited because such processing is restricted to within a slot. Interleaving reduces the likelihood that a user""s entire slot of information would be lost.
Referring to FIG. 3, there is shown the traditional 2-slot interleaving process for an IS-136 compliant TDMA system in the Full Rate mode; this interleaving scheme is referred to as the IS-136 1-4 interleaving scheme and is currently used in IS-136 TDMA communication systems. There are three users per frame, User A, user B, user C. Information to be transmitted by each of the users are stored in their respective buffers (i.e., temporary storage devices). That is, user A information is stored in buffer 301A, 302A, 303A etc . . . , user B information is stored in buffer 301B, 302B, 303B, etc . . . and user C information is stored in buffer 301C, 302C, 303C, etc . . . The capacity of each of the buffers is expressed in terms of time. For example, all of the buffers in FIG. 3 are 20 millisecond buffers meaning that all buffers transfer information at a certain rate and thus the time duration for transferring information determines the amount of information transferred. Similarly, each slot has an associated capacity that depends on the rate of transmission of information and the time duration of the slot. Note, the fractions shown (i.e., xc2xd) next to the lines going from a buffer to a time slot indicate a specific portion of the information stored in the buffer that is being transmitted during that time slot.
Thus, as shown in FIG. 3, interleaving is accomplished by transmitting xc2xd of each buffer in two different time slots. For example, for User A, xc2xd of the information in buffer 301A is transmitted in time slot 1 and xc2xd of the information is transmitted in time slot 4 of the same frame (i.e., frame 1). In buffer 302A xc2xd of the information therein is combined/intermingled with half of the information from buffer 301A and that aggregate of information is transmitted in slot 4 of frame 1. The other half of the information is combined/intermingled with information from buffer 303A and that aggregate of information is transmitted in time slot 1 of the next frame (i.e., frame 2). This process continues so that all of user A""s information is transmitted in slots 1 and 4. The information from user B is transmitted in the same manner as described above but using time slots 2 and 5. Similarly, the information from user C is also transmitted in the same manner but using time slots 3 and 6. Combining/intermingling of information refers to any process in which portions of different blocks of information are mixed forming an aggregate block of information. Thus, under adverse communication channel conditions where the entire information transmitted during a time slot has been corrupted, only xc2xd of the information from corresponding buffers are corrupted. When only xc2xd of the information from a buffer are corrupted (as opposed to all of the information being corrupted), the ability of current error correction techniques to successfully decode the information is enhanced.
There is a certain amount of delay in transmitting information when interleaving (as described above) is used. In the example discussed above, for user A, buffer 301A takes 20 milliseconds to be filled. Only xc2xd of the information stored in buffer 301A is transmitted in time slot 1. Buffer 302A takes another 20 milliseconds to be filled and then combined with half of the information from buffer 301A. It takes an additional 6.67 milliseconds to transmit the combined information from buffers 301A and 302A during time slot 4. Thus, there is a delay of 46.67 milliseconds (20 msec+20 msec+6.67 msec) before all of the information stored in buffer 301A is transmitted.
The effectiveness of the interleaving process discussed above directly depends on how the adverse effects of the communication channel affect the different time slots over which information is interleaved. For example, for user A, when the channel adversely effects information transmitted in time slot 1 and there is a high probability that information transmitted during time slot 4 is also adversely affected, then interleaving over these two time slots will not be effective. It is therefore desirable that the channel be decorrelated between the slots over which information is interleaved. A channel that is decorrelated between two particular time slots is such that any adverse affects occurring during one of the time slots are statistically independent and unrelated to any adverse affects occurring in the other time slot. Moreover, the occurrence of adverse effects in one time slot does not necessarily intimate that the other time slot will be adversely affected at all. The decorrelation feature implies that interleaving will provide better results the further apart (in time) information from a particular buffer is interleaved. However, interleaving over slots further apart increases the amount of delay needed to transmit information significantly beyond the standard 46.67 msec. For example, FIG. 4 shows an IS-136 1-7 interleaving scheme for half rate mode (6 users) in which half of the information in a buffer is transmitted in one slot and the other half of the information is transmitted in a seventh slot (i.e., slot 1 of the next frame). For user A, xc2xd of the information in buffer 402A is transmitted in slot 1 of frame 2. The remaining xc2xd of the information from buffer 402A is combined/intermingled with xc2xd of the information in buffer 403A, xc2xd the information from buffer 404A, xc2xd the information from buffer 405A and transmitted during slot 1 of frame 3. Note that all four buffers (402A-405A) must be filled before the second xc2xd of buffer 402A can be transmitted during slot 1 of frame 3. Thus, there is a 4xc3x9720 msec=80 msec. of delay incurred before the second xc2xd of buffer 402A can be transmitted during slot 1 of frame 3. Additionally, there is the 6.67 msec. of time needed to transmit the combined information during slot 1 of frame 2. Therefore, the total delay for a 1-7 interleaving scheme for half rate mode is 4xc3x9720+6.67=86.67 msec., viz., 40 msec. more than the traditional IS-136 1-4 interleaving scheme for full rate mode.
All of the interleaving schemes discussed above involve transmitting xc2xd of the information in a buffer over two slots. Although the schemes discussed above are the traditional means by which interleaving has been performed, it is not necessary that exactly half of each buffer be transmitted in different slots. It is desirable that the slots over which interleaving is done be chosen to be decorrelated in order for the interleaving to be effective. To increase the effectiveness of the interleaving, the time difference between the chosen slots should be as large as possible, but not too large so as not to significantly increase the time it takes to transmit an entire buffer of information.
Therefore, there is a need to develop an effective method of interleaving information that will result in a reasonable amount of delay in transmitting the information and yet exploit as much as possible any decorrelation feature present in the communication channel through which the information is being transmitted.
The present invention provides a method for interleaving information conveyed over a wireless communication system. Current interleaving techniques only consider equally dividing information blocks before interleaving. The current invention demonstrates, however, that information blocks can be unequally divided and furthermore, divided differently from information block to the next prior to interleaving in order to reduce delay time. A certain portion of a block of information to be conveyed by a user is transmitted during an assigned time slot. The remaining portion of the block of information is transmitted with portions of other blocks of information for the same user during at least one other time slot. The time slots are chosen such that the amount of time it takes to transmit the entire block of information does not increase significantly compared to a traditional method of interleaving.
In particular, the method of the present invention provides a method for interleaving a block of information for a certain user with at least another block of information for the user over a certain time period (e.g., a frame). After the block of information is stored in a temporary storage device (i.e., a buffer), a certain portion of the block of information is transmitted during one part (e.g., time slot) of the time period. At least one other block of information is stored in another storage device. A remaining portion of the block of information is subsequently combined and/or intermingled with a portion of the at least one other block of information and transmitted during at least one other part of the time period.